One of the most costly diseases that affects channel catfish (Ictalurus punctatus) and the channel catfish industry is enteric septicemia of catfish (ESC). ESC is caused by a gram negative enteric bacterium identified as Edwardsiella ictaluri (E. ictaluri). The exact economic impact of this disease is unknown but is estimated to cost the industry 30 million dollars per year. Aside from morbidity and mortality resulting from E. ictaluri infection, the disease has an indirect effect on growth due to the implementation of restricted feeding practices designed to prevent or slow infection rates. Our research has demonstrated that oral ingestion of the pathogen is one of the primary routes of infection and that withholding feed from fish when environmental conditions are conducive for infection can prevent or dramatically reduce ESC-related mortality. While effective in preventing disease, this practice dramatically reduces production due to lost feed days.
A more attractive method of disease control is through vaccination. Many attempts have been made to vaccinate fish against ESC using simple killed bacterin preparations. While effective in controlling bacterial diseases in salmonids, bacterin-type vaccines (delivered orally or by bath immersion) to control ESC have been for the most part unsuccessful. Failure to successfully immunize fish has been attributed to poor antigen uptake during bath immersion and/or the inability of killed vaccines to elicit cellular immunity necessary to protect fish against an intracellular bacterial pathogen such as E. ictaluri. To overcome these limitations, an attenuated E. ictaluri vaccine has been developed using prior published art/technology. The vaccine was developed by successive passage of a wild-type virulent E. ictaluri isolate on media containing increasing concentrations of rifamycin. Rifamycin resistance causes mutation in the lipopolysaccharide O-side chain and diminished virulence. This methodology for attenuation by serial passage is well-known and based on procedures used to produce live attenuated vaccines for brucellosis in cattle [Vet. Micro. 28, 171-188 (1991)].
Attenuated vaccines have advantages over killed bacterin vaccines for several reasons: 1) they are living and invasive, thereby facilitating vaccine uptake; 2) they establish low-grade infections resulting in the stimulation of cellular immunity and typically establish longer-lasting immunity. A disadvantage of live vaccines is that they can be associated with some level of virulence and can result in mortality if delivered to an immuno-incompetent or immuno-suppressed host.
U.S. Pat. No. 6,322,793 (Vanderheijden, et al.) provides an attenuated avirulent recombinant vaccine against channel catfish virus (CCV) comprising deletion of gene 50 which encodes a secreted glycoprotein. U.S. Pat. No. 6,350,454 (Thune) and U.S. Pat. No. 6,010,705 (Thune, et al.) disclose a live attenuated Pasteurella piscicida and E. ictaluri vaccine, respectively, for fish. Another such attenuated ESC vaccine as described above is commercially available and marketed under the trade name AQUAVAC-ESC™ (Intervet/Schering Plough, U.S. Pat. No. 6,019,981 (Klesius, et al.)). According to label directives, the vaccine is administered by bath immersions to catfish seven (7) days or older. To conform to industry practices, the vaccine is typically administered to catfish fry between 7-12 days of age-post hatch when fish are transferred from the hatchery to rearing ponds. The current vaccination strategy has three (3) primary drawbacks resulting in marginal efficacy in terms of disease protection and economic returns: 1. Fish are being vaccinated at a stage in development that prevents the development of an optimal immune response. Studies in other laboratories suggest 7-10 day old fry are not fully immunocompetent which marginalizes the full potential of the vaccination; 2. Optimal immunity resulting from vaccination does not coincide with the onset of ESC because of the seasonal occurrence pattern of the disease. Studies conducted in our laboratory have demonstrated that immunity resulting from vaccination with AQUAVAC-ESC™ is short-lived and begins to wane after 1 month. In a typical production season, fry are produced and vaccinated approximately 2-3 months prior to observance of optimal environmental temperature windows favorable for the emergence of ESC. This temporal relationship between vaccination and the onset of disease dramatically decreases the efficacy of the vaccination; and 3. Poor survival of vaccinees prior to the emergence of ESC increases the unit cost of vaccination and decreases net returns on vaccine investment. It is typical for commercial producers to experience on average 20% mortality in newly-stocked fry and young fingerlings from a variety of non-disease related causes. These types of losses are commonly referred to as black-hole losses and are thought to occur shortly (within 3 weeks) after fry are transferred from the hatchery to the pond. In addition, it is common for 3-5% of the ponds used in the production channel catfish fingerlings to experience 100% mortality due to unknown causes or oxygen depletions. These losses increase the unit cost of vaccination, since the cost of vaccinating fry that do not survive the initial stages of production must be distributed across the surviving vaccinates. In addition, with current vaccination practices, much apprehension is generated due to the potential occurrence of vaccine-induced mortality. In laboratory trials, bath vaccination with live attenuated E. ictaluri vaccines has resulted in significant mortality starting 3-7 days post-vaccination. These responses have been variable and are thought to be related to the young age of fry at vaccination. Other trials have shown that these adverse reactions to vaccination decreased with increasing fry age. No adverse effects to vaccination have been shown in fish over 30 days of age. Therefore, oral in-pond vaccination of fish, specifically of catfish, utilizing the present invention will not only improve efficacy as stated above but also increase the safety of vaccination.
A need exists in the field of preventative and protective fish vaccination for a new and efficient methodology for vaccinating fish to protect against such diseases. The present invention provides such a set of isolets, method of administration, and apparatus for delivery of vaccine. The present invention in a preferred embodiment utilizes at least one novel live attenuated Edwardsiella ictaluri isolate for vaccination of fish and a new and distinctive method of oral delivery and administration of any vaccine to fish, specifically catfish, for preventing disease and decreasing mortality. The present invention further provides an apparatus designed for delivering any vaccine utilizing the new method of oral vaccine delivery.